1. Field of the Invention
The present invention relates to a control system for actively distributing driving torques applied to wheels in four-wheel drive automotive vehicles, and specifically to an electronic active torque-split control system for actively controlling the driving torque distribution between front wheels and rear wheels in four-wheel drive vehicles.
2. Description of the Background Disclosure
Recently, there have been proposed and developed various four-wheel drive vehicles in which driving torque transmitted to front-wheels is variably and actively controlled by the magnitude of engaging force created by a transfer clutch disposed in a transfer device which is connected to both front and rear propeller shafts, so as to achieve torque distribution between a front-wheel driving torque and a rear-wheel driving torque. One such four-wheel drive vehicle has been disclosed in Japanese First Publication (Tokkai) Showa 62-265030. In such a conventional four-wheel drive vehicle, a driving torque transmitted to a front-wheel would be basically determined in proportion to the rotational speed difference between front and rear wheel speeds. The proportional gain required for arithmetically determining the driving torque transmitted to the front-wheel is generally derived on the basis of a lateral acceleration exerted on the vehicle body such that the proportional gain is reduced in accordance with an increase in the lateral acceleration. Such a driving torque distribution control achieved between front and rear wheels is in general referred to as an `active torque-split control`. The previously noted conventional active torque-split control wherein a driving torque distribution is determined depending upon both a rotational speed difference between front and rear wheels and a lateral acceleration exerted on the vehicle body, can provide a superior steering characteristics and a high drivability regardless of a frictional coefficient of road surfaces, namely a low frictional road surface such as wet or icy roads and a high frictional road surface such as dry pavement, since the behaviour of the vehicle is controlled to be always maintained in a substantially neutral steer. Such a prior art active torque-split control system can provide sufficient traction and cornering force on turns.
However, in the aforementioned conventional torque-split control, the vehicle is maintained in a substantially neutral steer and a rate of driving torque distribution between front and rear wheels is gradually and moderately varied depending upon the rotational speed difference between front and rear wheels and the lateral acceleration of the vehicle body, even when the vehicle is operated in a critical state with regard to tires and suspension systems, such as high-speed driving on curves. For this reason, it is difficult for the driver to quickly and predictively feel that the vehicle is close to the critical state, during hard turns with a great lateral acceleration, for example.